This invention relates to an electrodeposition coating composition of cathode-depositing type.
Certain types of resins having basic groups are dissociated into positively charged resin particles in an aqueous solution or dispersion thereof. When electrodepositing the solution or dispersion, these resins are deposited on the cathode. Cathode-depositing electrodeposition coating compositions have an important advantage over anode-depositing coating compositions containing a neutralizate of acid group-bearing resins in that the former eliminates the defects of the latter, i.e., dissolution of metallic substrates into the coating bath and other problems associated therewith.
It is known that a cathode-depositing electrodeposition coating composition may be prepared by introducing amino groups into carbon-to-carbon double bond-containing synthetic polymers such as liquid polybutadiene polymers and then neutralizing with an acid. See, Japanese Laid Open Patent Application Nos. 51-119727, 52-147638 and 53-16048.
This composition produces a cured film having excellent properties mainly through oxidative polymerization of unsaturated bonds possessed by the resin. This composition, however, requires relatively high baking temperatures when the coating film thereof is cured within a practical length of time. Low baking temperatures may be achieved by adding a drier compound such as water-soluble manganese salts. See, Japanese Laid Open Patent Application No. 53-142444. In this case, relatively large amounts of drier compound must be added. As a result, electrodeposition characteristics of the coating composition, such as capability of forming smooth and uniform films on the substrate are often impaired.
Low baking temperatures may also be achieved by introducing acrylic or methacrylic double bonds into the resin. See, Japanese Laid Open Patent Application No. 56-151777. In this case, a cathode-depositing electrodeposition coating composition which forms a cured film having excellent properties at a baking temperature as low as 160.degree. C. may be obtained by incorporating an amount of water-soluble manganese salts.
Despite these and other attempts, the prior art electrodeposition coating compositions of the above type still have various problems. Since the oxidative polymerization reaction of unsaturated film-forming polymers takes place first from the surface of the resulting film, wrinkles are often formed or the interior of the film is not fully cured.
It is a principal object of the present invention to provide a cathode-depositing electrodeposition coating composition which eliminates shortcomings of the prior art compositions.